The present invention relates generally to plasma ashing processes for selectively removing photoresist, polymers and residues from semiconductor substrates. More particularly, the process relates to plasma ashing semiconductor substrates including low k dielectric surfaces.
Ashing is a plasma mediated stripping process by which photoresist, polymer and/or residues are stripped or removed from a substrate upon exposure to the plasma. Ashing generally occurs after an etching process has been performed in which the photoresist material is used as a photomask for etching a pattern into the substrate. Additionally, the ashing process may be performed for removal of misaligned resist patterns (xe2x80x9crework wafersxe2x80x9d) and in lift-off processes. The process steps occurring prior to ashing may modify the surface of the photoresist, form polymers and/or form residues. It is highly desirable when ashing that complete removal of the photoresist, polymer and/or residues occur as quickly as possible without loss or modification of any of the materials comprising the substrate. Minimizing the loss or modification is a constant challenge.
It is important to note that ashing processes significantly differ from etching processes. Although both processes may be plasma mediated, an etching process is markedly different in that the plasma chemistry is chosen to permanently transfer an image into the substrate by removing portions of the substrate surface through openings in a photoresist mask. The plasma generally includes high energy ion bombardment at low temperatures to remove portions of the substrate. Moreover, the portions of the substrate exposed to the ions are generally removed at a rate equal to or greater than the removal rate of the photoresist mask. In contrast, ashing processes generally refer to selectively removing the photoresist mask and any polymers or residues formed during etching. Transferring a permanent image into the underlying substrate is not the purpose of this step. Accordingly, the ashing plasma chemistry is much less aggressive than etching chemistries and generally is chosen to remove the photoresist mask layer at a rate much greater than the removal rate of the underlying substrate. It is desirable to generate low energy ions in the plasma for removing the photoresist, polymers and/or residues without causing damage to the substrate. Moreover, most ashing processes heat the substrate to temperatures greater than 200xc2x0 C. to increase the plasma reactivity. Thus, etching and ashing processes are directed to removal of significantly different materials and as such, require completely different plasma constituents, chemistries and processes. Successful ashing processes are defined by the photoresist, polymer and residue removal rates without affecting or removing layers comprising the underlying substrate.
Ashing selectivity is defined as the relative removal rate of the photoresist compared to the underlying layer. It is preferred to have an ashing selectivity of at least 50:1, wherein the photoresist is removed at least 50 times faster than the underlying substrate. More preferably, the ashing selectivity is much greater than 100:1.
During plasma ashing processes, it is important to maintain a critical dimension (CD) for the various features within a tightly controlled specification as well as promote proper underlayer surface conditions for successful metal filling in the process steps occurring after photoresist, polymer, and residue removal. Small deviations in the patterned profiles formed in the underlayers can adversely impact device performance, yield and reliability of the final integrated circuit. Traditionally, the ashing plasma has been generated from oxygen-containing gases. However, it has been found that oxygen-containing plasmas readily damage certain materials used in advanced integrated circuit manufacture. For example, oxygen-containing plasmas are known to raise the dielectric constant of low k dielectric underlayers during plasma processing. The increases in dielectric constant affects, among others, interconnect capacitance, which directly impacts device performance. Moreover, the use of oxygen-containing plasmas is generally less preferred for advanced device fabrication employing copper metal layers.
In order to overcome some of these problems, oxygen-free plasma chemistries have been developed. Oxygen-free plasmas can be used to effectively remove photoresist, polymers and residues from substrates containing low k dielectric materials without causing significant physical damage to the low k dielectric layer. Oxygen-free plasmas are typically generated from a hydrogen and nitrogen gas mixture that may further contain fluorine gases. However, in some cases it has been found that the use of oxygen-free plasmas containing nitrogen may alter and/or affect the chemical, mechanical and electrical properties of underlying substrate.
Accordingly, it is highly desirable to have an ashing process that completely and rapidly removes the photoresist, polymer and residues without affecting or removing the underlying surface materials, including low k dielectric surfaces.
A plasma ashing process and apparatus for selectively removing photoresist and/or post etch residues from a semiconductor substrate is described. In one embodiment, the ashing process includes placing a wafer into a processing chamber, wherein the wafer comprises a surface having the photoresist and/or post etch residues thereon; generating a reduced ion density plasma in a plasma generation region at a pressure of at least 2 torr greater than the processing chamber pressure; and exposing the wafer surface having the photoresist and/or post etch residues thereon to the reduced ion density plasma to selectively remove the photoresist and /or post etch residues from the surface and leave the surface substantially the same as before exposing the substrate to the reduced ion density plasma.
In another embodiment, a process for reducing ion density in a downstream plasma asher includes flowing a gas mixture into a plasma-generating region at a predetermined pressure; and exposing the gas mixture to an energy source sufficient to form a plasma in the plasma generating region at the predetermined pressure, wherein the predetermined pressure is selected to reduce the ion density in the plasma. The process may further include discharging the reduced ion density plasma into a processing chamber, wherein the predetermined pressure is selected to create a pressure differential greater than 2 torr between the plasma generating region and processing chamber, the pressure being greater in the plasma generating region. The process may further include exposing a surface having a photoresist and/or post etch residues thereon to the plasma to selectively remove the photoresist and /or post etch residues from the surface, wherein the surface comprises a low k dielectric material and is substantially the same as before exposing the surface to the reduced ion density plasma. Preferably, the predetermined pressure in the plasma-generating region ranges in an amount from greater than 2 torr to atmospheric pressure.
A downstream plasma asher apparatus includes a processing chamber; a plasma generating region comprising a gas inlet, a plasma tube coupled to the gas inlet and a discharge opening; a conduit in communication with the processing chamber and the discharge opening of the plasma generating region for discharging a plasma formed in the region into the processing chamber; and an orifice disposed in the conduit, wherein the orifice narrows an opening of the conduit to create a pressure differential greater than 2 torr between the plasma tube and the processing chamber, wherein the pressure is greater in the plasma tube. In one embodiment, the downstream plasma asher includes a microwave energy source in communication with the plasma generating region for exciting a gas mixture to form a reduced ion density plasma. In another embodiment, the downstream plasma asher includes a radio frequency energy source in communication with the plasma generating region for exciting a gas mixture to form a reduced ion density plasma. Preferably, the orifice comprises a variable aperture for adjusting the pressure in the plasma-generating region.